I'm also trying to discourage the use of `.asc`. Perhaps we should remove that one from Awestruct outright.

@ge0ffrey I don't mind recommending one and putting the others in footnotes. The reason there are so many file types supported is because that's the current state of things. We are attempting to converge on a favorite, which at the moment seems to be `.adoc`, but there are many docs out there already that use the other extensions.

The other thing to consider is that the file types are actually managed in Tilt, not in Awestruct. We are overriding the implementation in Awestruct atm, so we also list the extensions in Awestruct, but that's just to cover what Tilt already recognizes. I think we can *deregister* extensions from Tilt, though. Perhaps open an issue for this?